Text 796, 80 rader
Skriven 2006-07-07 15:55:41 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 784
===============
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 784 July 7, 2006  by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi        www.aip.org/pnu

RED OXYGEN.  A new evolutionary crystallography algorithm predicts
the structure of  crystals under a range of extreme pressure and
temperature conditions on the basis of the chemical composition
alone.  One of these crystals would be a form of red-colored oxygen.
Predicting crystal structures is difficult even for simple solids,
partly because of the task of sorting among the astronomical number
of possible ways given atoms can compose a basic repeatable unit
cell. Artem Oganov, a scientist at ETH Zurich, and Colin W. Glass, a
PhD student, approach the problem by combining electronic structure
calculations and a specifically developed evolutionary algorithm. In
exploring the myriad atomic arrangements, they proceed in a
step-by-step, continual-optimization fashion that avoids
configurations less likely to succeed. This makes the algorithm very
efficient and allows the researchers to make certain specific
predictions.  One example is calcium carbonate (CaCO3) at very high
pressures.  Oganov's team for the first time predicted two new
stable structures for this mineral. By now, both structures have
been confirmed in experiments by Japanese colleagues. Oganov and
Glass have also solved the structures crystalline oxygen at high
pressure. Oxygen is unique from the chemical point of view.  The
only magnetic molecular element known, under pressure it loses its
magnetism and turns red. The structure of red oxygen, which remained
unknown for a long time, seems to be finally solved and turns out to
be unique; that is, it does not manifest itself in any other
element.  At even higher pressure oxygen is known to turn black in
color and become superconducting, which happens because of the
increased interactions between the O2 molecules.  The ETH
researchers also predict a new stable phase of sulphur and several
new metastable forms of carbon. (Journal of Chemical Physics, 28
June 2006; lab website at http://olivine.ethz.ch/~artem/ ; ETH
Laboratory of Crystallography, 41(0)44 632 37 52,
a.oganov@mat.ethz.ch)

SQUEEZED LIGHT AND GRAVITY WAVES.  A proven method for reducing the
noise in high-precision optical measurements will soon be applied to
the search for gravitational waves.   The most likely way such waves
will be detected is by observing their subtle effects on suspended
mirrors in detectors like the Laser Interferometer
Gravitational-wave Observatory (LIGO).  At LIGO, laser light is
split into two beams which reflect many times from mirrors suspended
at the ends of two long pipes positioned at right angles.  The two
beams are brought back together to form an interference pattern.
This procedure is adjusted so that a photodetector is positioned at
a null in the pattern; that is, it normally sees no photons coming
its way.  The plan is that a passing gravity wave would ever so
slightly move the suspended mirrors in the two pipes
(which are otherwise insulated from ordinary kinds of vibration)
relative to each other, which in turn would disturb the interference
pattern. Suddenly the photodetector would record photons, heralding
a gravity wave. One problem with this scheme is "shot noise," the
quantum-based uncertainty in our knowledge of how many photons are
present in a laser beam at any moment.  Fluctuations in photon
number could trigger a false positive reading.  Physicists at the
Max Planck Institute (Hannover) and the University of Hannover are
hoping to reduce the quantum noise inherent in this interferometric
approach to gravity wave detection by squeezing light. Squeezed
light is produced when quantum noise in one or the other of  two
complementary variables describing a light beam (such as phase and
amplitude) is greatly reduced at the expense of the other by sending
the light through (a series of) special optical crystals.  The use
of squeezed light reduces quantum noise in a number of
optoelectronic applications.  Usually the squeezed light approach is
applied at megahertz frequencies, but the Hannover researchers have
for the first time gotten it to work at all the detection
frequencies pertinent for LIGO including frequencies below a hundred
hertz, the expected frequency range of gravitational waves arriving
from some distant coalescing black holes in the universe.  According
to Henning  Vahlbruch(henning.vahlbruch@aei.mpg.de) a squeezed-light
control scheme would help reduce noise and raise the sensitivity of
gravity wave detectors.  (Valbruch et al., Physical  Review Letters,
7 July 2006;  website at
http://www.geo600.uni-hannover.de/~schnabel/ )

---
 * Origin: Big Bang (1:106/2000.7)